Batteries and battery systems

ABSTRACT

A battery includes a cathode having an interior surface and an exterior surface, and defining a cavity and two open ends; a separator disposed adjacent to the interior surface of the cathode; an anode disposed adjacent to the separator and inside the cavity; an air-permeable, liquid-impermeable barrier layer disposed adjacent to the exterior surface of the cathode, and defining an exterior surface of the battery; two end members connected to the open ends of the cathode; and an anode current collector extending through the two end members. Methods designed such a battery is also disclosed.

CLAIM OF PRIORITY

This application claims priority under 35 USC §119(e) to U.S. Ser. No.10/060,701, filed Jan. 30, 2002, and entitled “Batteries and BatterySystems”, now U.S. Pat. No. 7,065,617, which claims priority from U.S.Provisional Patent Application Ser. No. 60/265,822, filed on Feb. 1,2001, and entitled “Battery”, the entire contents of which are herebyincorporated by reference.

TECHNICAL FIELD

This invention relates to batteries and battery systems.

BACKGROUND

Batteries are commonly used electrical energy sources. A batterycontains a negative electrode, typically called the anode, and apositive electrode, typically called the cathode. The anode contains anactive material that can be oxidized; the cathode contains or consumesan active material that can be reduced. The anode active material iscapable of reducing the cathode active material. In order to preventdirect reaction of the anode material and the cathode material, theanode and the cathode are electrically isolated from each other by aseparator.

When a battery is used as an electrical energy source in a device,electrical contact is; made to the anode and the cathode, allowingelectrons to flow through the device and permitting the respectiveoxidation and reduction reactions to occur to provide electrical power.An electrolyte, for example, potassium hydroxide in contact with theanode and the cathode contains ions that flow through the separatorbetween the electrodes to maintain charge balance throughout the batteryduring discharge.

In a metal-air electrochemical cell, oxygen is reduced at the cathode,and a metal is oxidized at the anode. Oxygen is supplied to the cathodefrom the atmospheric air external to the cell through one or more airhole(s) in the cell can.

To prolong battery life, it is desirable that the cathode be exposed toair flow when in use, and isolated from air flow when not in use. Duringuse, it is desirable to provide uniform and sufficient air access to thecathode to provide, for example, uniform discharge of the activematerials and/or a relatively high discharge voltage profile.

SUMMARY

This invention relates to batteries and battery systems.

In one aspect, the invention features a non-hermetically sealed,electrochemical power source having a first electrode, a secondelectrode, a separator between the first electrode and the secondelectrode, and a membrane in fluid communication with an environmentexternal to the battery, the second electrode being between theseparator and the membrane. The membrane includes a first portion havinga density different than a second portion of the membrane.

In another aspect, the invention features a non-hermetically sealed,electrochemical power source having a first electrode, a secondelectrode, a separator between the first electrode and the secondelectrode, and a membrane in fluid communication with an environmentexternal to the battery, the second electrode being between theseparator and the membrane. The membrane has a first portion having aporosity different than a second portion of the membrane.

In another aspect, the invention features a non-hermetically sealed,electrochemical power source including a first electrode, a secondelectrode, a separator between the first, electrode and the secondelectrode, and a material in fluid communication with an environmentexternal to the battery, the second electrode being between theseparator and the material. Gas permeability across a first portion ofthe material is different than gas permeability across a second portionof the material.

In another aspect, the invention features a non-hermetically sealed,electrochemical power source having a first electrode, a secondelectrode, a separator between the first electrode and the secondelectrode, and a membrane in fluid communication with an environmentexternal to the battery, the second electrode being between theseparator and the membrane. The membrane has a first portion having amass transport resistance different than a second portion of themembrane.

In another aspect, the invention features a non-hermetically sealed,electrochemical power source having a first electrode, a secondelectrode, a separator between the first electrode and the secondelectrode. The separator comprises a first portion having a masstransport resistance different than a second portion of the separator.

Embodiments of the aspects of the invention may include one or more ofthe following features. The power source further includes a containerhaving an air access opening, wherein the first portion is adjacent tothe air access opening and has a density higher than the second portion.The second portion is farther from the air access opening than the firstportion is from the air access opening. The first portion is alignedwith the air access opening. The first portion has an area greater thanthe area of the air access opening. The membrane is permeable to a gas.The first and second portions are integrally formed as one component.The membrane includes polytetrafluoroethylene. The power source is ametal-air cell. The power source further includes a container having aplurality of air access openings, wherein the membrane has uniform masstransport resistance to a gas flowing through the air access openings.

In another aspect, the invention features a battery cartridge includinga housing having a plurality of air access openings configured toselectably control flow of a gas into the housing, and anelectrochemical cell in the housing, the cell having a top surfaceadjacent to the air access openings and a side surface. The openings arepositioned over the side surface of the cell, and the housing is free ofopenings completely over the top surface of the cell.

Embodiments may include one or more of the following features. Thecartridge includes two adjacent electrochemical cells in the cartridge,the cells defining a gap; therebetween, wherein the openings are onlypositioned over the side surfaces of the cells and over the gap. Thecartridge includes two adjacent electrochemical cells in the cartridge,the cells defining a gap therebetween, wherein the openings are onlypositioned over the side surfaces of the cells or over the gap. Theopenings are off-centered away from the cell. The openings are centeredover the side surface of the cell.

Embodiments may include one or more of the following advantages. Thebatteries and/or battery systems may have relatively long activatedshelf life. The batteries and/or battery systems may have relativelyhigh utilization of active materials. The batteries and/or batterysystems, may have enhanced uniformity in the current distribution ordensity. As a result, relatively uniform flooding and/or improvedutilization of active materials can be achieved. The batteries and/orbattery systems can be useful for high current or rate applications ordevices.

In one aspect, the invention features a battery, such as a metal-airbattery, having a cathode having an interior surface and an exteriorsurface and defining a cavity, a separator disposed adjacent to theinterior surface of the cathode, an anode disposed adjacent to theseparator and inside the cavity, and an air-permeable,liquid-impermeable barrier layer disposed adjacent to the exteriorsurface of the cathode, the barrier layer defining an exterior surfaceof the battery. Thus, the cathode serves as a container or a case forthe battery.

The cathode can define two open ends. Two end members can be connectedto the open ends of the cathode, and an anode current collector canextend through the two end members.

Embodiments of the invention may include one or more of the followingfeatures. The cathode includes a current collector, and the batteryfurther includes a conductive tab connected to the current collector.The current collector is connected together to form a seam, and theconductive tab is connected to the current collector along the seam. Thebattery further includes a sealant disposed over the conductive tab. Thebarrier layer includes. polytetrafluoroethylene. The end members areconfigured to mate with the cathode and align with the cathode, along alongitudinal axis of the battery. The battery further includes aconductive tab connected to the anode current collector. The end membersand the cathode are connected by a sealant. The end members comprise anelectrically-insulating material, such asacrylonitrile-butadiene-styrene. The separator is glued to the cathode.The barrier layer is laminated to the cathode.

The cathode can have a substantially rectangular, substantially squarecross, substantially triangular, or substantially circular crosssection.

The battery can be dimensioned to fit inside a housing adapted to manageair flow into and out of the housing.

In another aspect, the invention features a method of making a battery.The method includes placing a first layer adjacent to a cathode, thefirst layer being electrically-insulating, placing a second layeradjacent to the cathode, the second layer being air-permeable andliquid-impermeable, and forming the cathode wherein the second layerdefines an exterior surface of the battery. The method can also includemating the formed cathode with two end members and connecting the endmembers to the cathode with a sealant, wherein mating the formed cathodewith the two end members includes placing an anode current collectorthrough the end members.

Embodiments of the invention may include one or more of the followingfeatures. Placing the second layer adjacent to the cathode includeslaminating the second layer to the cathode. Placing the first layeradjacent to the cathode includes gluing the first layer to the cathode.Forming the cathode includes defining a cavity with the cathode, and themethod further includes placing an anode in the cavity. The cathodeincludes a current collector, and forming the cathode further includesconnecting the current collector together to form a seam. The methodfurther includes connecting a conductive tab to the current collector.The method further includes placing a sealant on the conductive tab.

Other features and advantages of the invention will be apparent from thedescription of the preferred embodiments thereof and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an embodiment of a battery cartridge.

FIG. 2 is an exploded perspective view of the battery cartridge of FIG.1, shown with electrochemical cells.

FIG. 3 is a plan view of the battery cartridge of FIG. 1.

FIG. 4 is a cross sectional view of the battery cartridge of FIG. 3,taken along line 4-4

FIG. 5 is a detailed view of the battery cartridge of FIG. 4.

FIG. 6 is a perspective view of an embodiment of an electrochemicalcell.

FIG. 7 is partially cut-away view of the cell of FIG. 6.

FIG. 8 is a cross-sectional view of the cell of FIG. 6

FIG. 9 is a detailed view of the cell of the FIG. 8.

FIG. 10 is a partial perspective view of an embodiment of a cell system.

FIG. 11 is a detailed view of the cell system of FIG. 10.

FIG. 12 is a schematic diagram of a cross section of an embodiment of acell pack.

FIG. 13 is a schematic diagram of a portion of the cell pack of FIG. 12.

FIG. 14 is a plot of oxygen concentration as a function of number ofcolumns per plenum.

FIG. 15 is a flow chart of an embodiment of a method for designing airaccess openings.

FIGS. 16A, 16B, 16C, and 16D are illustrations of embodiments ofsimulation patterns of modified barrier layers.

FIG. 17 is a finite element mesh for a 45 degree portion of a buttoncell.

FIGS. 18A and 18B are simulated oxygen distributions at 5 mA appliedcurrent for a modified and unmodified barrier layer, respectively.

FIG. 19 is a plot of limiting current as a function of final porosityfor the patterns of FIGS. 16A-16D.

FIG. 20 is a plot of modified shelf life/unmodified shelf life as afunction of final porosity for the patterns of FIGS. 16A-16D.

FIG. 21 is a plot of maximum current density/minimum current density forthe patterns of FIGS. 16A-16D.

FIG. 22 shows plots of standard deviation and Max/Min as a function ofincreases, in air access opening diameter.

FIG. 23 shows plots of changes in limiting current and shelf life, andchanges in current density standard deviation as a function of changesin air access opening diameter.

FIG. 24 shows plots of changes in activated life and changes in limitingcurrent as a function of changes in air access opening diameter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1-5, a rectangular prismatic battery cartridge orpack 100 includes a casing 102, here, shown with a plurality ofmetal-air cells 104 (e.g., three) inside the casing. Casing 102 isshaped as a rectangle having a wall extending around the periphery ofthe rectangle. Cartridge 100 further includes a valve frame 106 fittedover casing 102, a moveable air valve 108, and a fixed air valve 110.Valves 108 and 110 each has a pattern of air access openings or holes112 that can align (e.g., complete overlap), partially align (e.g., someoverlap), or misalign (e.g., no overlap) with each other when moveableair valve 108 is slid in a plane defined by valve 108 (FIG. 1, arrow A).

When cartridge 100 is used in a device, such as a cellular telephone,moveable air valve 108 can be positioned, e.g., slid, relative to fixedair valve 110 according to a mode of operation of the device. Forexample, when the device is in an “off” mode, holes 112 of valves 108and 110 are completely or substantially completely misaligned. Cartridgetightly or sufficiently seals cells 104 from the environment. Air flowto cells 104 is restricted to enhance the service life of the cells,e.g., by protecting the cells from self-discharge and/or by minimizingpremature degradation of battery materials from excessive exposure toair. When the device is in a “standby” mode, holes 112 of valves 108 and110 are partially aligned. Air flow to cells 104 is balanced, forexample, so that a sufficient amount of air may reach the cells tosatisfy the device's power and/or power up requirements during thestandby mode. When the device is in an “on” mode (e.g., “talk” mode fora cellular telephone), holes 112 are completely or substantiallycompletely aligned. In this mode, sufficient air reaches cells 104 toallow the cells to operate at full or substantially full levels.

In some embodiments, pack 100 further includes a low friction, absorbentlayer (e.g., Whatman paper (P3) having a non-woven polyamide fiberfabric) that extends across casing 102 between movable air valve 108 andcells 104. Pack 100 can further include a porous layer (e.g., apolyurethane open-cell foam) between the absorbent layer and cells 104.Pack 100 can further include an absorbent layer with a hydrophobic vaporbarrier (e.g., Whatman paper (P3) having a polytetrafluoroethylene film)between the porous layer and cells 104. Other embodiments of batterypacks or cartridges, including methods of use and operation, aredescribed in commonly-owned U.S. Ser. No. 09/693,010, filed Oct. 20,2000, and entitled “Battery Systems”, now U.S. Pat. No. 6,384,574,hereby incorporated by reference in its entirety.

Cells 104 are prismatic metal-air electrochemical cells configured to beplaced inside cartridge 100. Referring to FIGS. 6 and 7, metal-air cell104 includes a cathode assembly 22, a bottom end member 24 connected toan end of assembly 22, and a top end member 26 connected to the otherend of assembly 22. Cathode assembly 22 includes a cathode 28 formed ona current collector 30, a separator 32 glued to an interior side ofcathode 28, and a barrier layer 34, e.g., a polytetrafluoroethylene(PTFE or Teflon®) layer, wrapped around an exterior side of cathode 28.Cathode assembly 22 is formed, e.g., wrapped, to define a cavity 36.Cell 104 further includes an anode 38 and an anode current collector 40disposed in cavity 36. Anode current collector 40 extends from bottomend member 24, through cavity 36, and through top end member 26. Anegative tab 42 is connected to anode current collector 40, and apositive tab 44 is connected to cathode current collector 30. When cell104 is fully assembled, barrier layer 34 defines an exterior surface ofthe battery. That is, cell 104 does not include a housing, such as acylindrical metal container or can, exterior to barrier layer 34. Duringuse, oxygen from the air flows through holes 112, passes through barrierlayer 34, contacts cathode 28 and is reduced as part of the cells'electrochemical reactions to provide electrical energy. Cells 104 aredescribed in more detail below and in incorporated-by-reference U.S.Ser. No. 60/265,822.

Modified Barrier Layer

Without wishing to be bound by theory, it is believed that in metal-airpacks, there are two mass transport resistances, e.g., for materialssuch as oxygen and water. One transport resistance is at the pack leveland the other at the cell level. At the pack level, the mass transportresistance, e.g., to oxygen, is in part a function of the plenum depth(the distance between cells 104 and moveable air valve 108), andplacement and area of the air access holes. At the cell level, activatedshelf life for the pack depends on water transport into and/or out ofthe cells. The vapor pressure of water in the cathode (which isdependent on the concentration of the electrolyte) and/or the relativehumidity of the air in contact with the membrane are also variables thataffect the water vapor flux. Generally, water transport out of the cellscan shorten a cell's activated shelf life. Therefore, in some cases, itis preferable to maximize the mass transport resistance to water tominimize water flux.

Furthermore, it is believed that in embodiments in which barrier layer34 is uniform around cell 104, the oxygen concentration at the top ofthe cell at the barrier layer/cathode interface is about at least 30%greater than the oxygen concentration at the two sides of the cell. Thismay be because the top of the cell is closer, i.e., a shorter diffusionpath, to the air access holes than the sides. This difference in theoxygen transport paths may result in a non-uniform current distributionat the cathode, which in turn can result in a non-uniform reaction planemovement in the anode and lowered anode utilization. Reducing thisnon-uniformity in oxygen partial pressure can provide a relatively moreuniform current distribution and therefore increased anode utilization.Thus, it is believed that to enhance performance, uniform oxygen accessis preferably maximized and/or water transport is preferably minimized.

Accordingly, in embodiments, barrier layers 34 of cells 104 are modifiedor altered to affect, e.g., increase or decrease, the mass transportresistance of the cells. For example, altering the properties of barrierlayer 34 at the top of the cells can allow less water vapor to enterand/or exit the Teflon barrier layer or the plenum, thereby allowing thecells to retain water relatively longer. Altering the top side can alsoenhance the cell performance because the oxygen partial pressure can berelatively more uniform on the sides of cells 104, thereby enhancinganode utilization.

In some embodiments, one or more portions of barrier layer 34 aremodified relative to another portion(s) of the barrier layer to adjustthe rate of flow of materials, such as oxygen and water, through thebarrier layer. Portion(s) of barrier layer 34 can have different masstransport resistance or permeability to selected material(s) than otherportion(s) of the barrier layer. Portion(s) of barrier layer 34 can havedifferent porosity than other portion(s) of the barrier layer.Portion(s) of barrier layer 34 can have different apparent density thanother portion(s) of the barrier layer. In embodiments, one or moreportions of barrier layer 34 are not uniform around the cells.

Numerous methods can be used to modify, e.g., increase or decrease,properties of barrier layer 34, such as the mass transport resistance ofa material, e.g., water and/or oxygen, through the layer. In someembodiments, barrier layer 34 is mechanically worked. Barrier layer 34can be, for example, thumped, compressed, deformed, and/or stretched.Thumping and/or compressing can be, performed using anappropriately-sized die, such as a blunt point or an awl. For example,mechanically working the barrier layer can result in variable porosityin which the porosity of the worked areas is lower than areas notworked, in variable density in which the density of the worked areas ishigher than areas not worked, and/or in variable mass transportresistance in which transport resistance increases in worked areasrelative to areas not worked.

The degree of modification can vary and can be controlled, for example,by controlling the amount of work applied. In embodiments, relative toan area that is not altered,;the altered areas can have a lowerporosity, e.g., 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, or 10% of theporosity of the unaltered area. The unaltered and altered areas can havesimilar differences in terms of mass transport resistance and/ordensity.

Other methods of modifying the mass transport resistance of a materialinclude chemical methods. For example, to increase mass transportresistance, a selected area can be modified by applying, e.g., painting,spraying, or coating, with a material that hinders, e.g., water and/oroxygen flux. Examples of suitable materials include an epoxy, afluoro-liquid (such as Kel-F oil), or a glue. Selected areas can beinjected with a material, such as a chemical precursor or reactant, thatobstructs or clogs the areas. For example, the injected material canreact with the anode to form ZnO and clog barrier layer 34.

In some embodiments, the thickness of barrier layer 34 can be modified,e.g., to affect mass transport resistance. Multiple layers of barrierlayer 34 can be used in selected areas, e.g., at the top of the cells,to increase the mass transport resistance of the areas. Barrier layer 34can be formed, e.g., extruded, with selected areas having a thicknessdifferent, e.g., greater, than other areas. The extra thickness ofbarrier layer needed to obtain similar results as a 50% decrease inporosity can be estimated. Mass transport resistance for vapor or gasphase resistance is given as the ratio of the gas diffusion path length(the barrier layer thickness) to the gas phase diffusion coefficient,mathematically expressed as

$R_{MT} = \frac{\delta}{D\; ɛ^{1.5}}$where R_(MT) is the mass transport resistance, δ is the thickness of thebarrier layer and D_(ε) ^(1.5) is the effective diffusion coefficient inthe barrier matrix. To achieve the same improvement, e.g., in the shelflife, that is obtained by the 50% reduction in porosity, the thicknessof the barrier layer should be increased to approximately 2.8 times theunaltered layer.

Various combinations of methods of modifying the mass transportresistance of barrier layer 34 can be used. That is, the selected areascan be modified by mechanical methods, chemical methods, and/or any ofthe methods described herein. For example, selected areas can bemodified by using multiple layers of barrier layer 34 that is coatedwith material that increases mass transport resistance, or the areas canbe modified by mechanical work and chemical methods.

Various areas of cells 104 can be modified. The areas to be modified aregenerally selected to maximize cell performance by maximizing uniformutilization of active materials and minimizing water loss. The modifiedareas can include the entire top surface of cells 104 or selectedportions of the top surface. Along selected areas, e.g., the topsurface, the degree of modification can be varied. For example, the masstransport resistance can be graded from one side of cell to the otherside of the cell, e.g., in which the mass transport resistance is at aminimum at the middle of the top surface but at a maximum at near thesides. The sides and/or bottom of cells 104 can be modified as describedherein.

In other embodiments, modification of the barrier layer as describedherein can be applied to systems other than cells 104, such as otherbattery systems. In general, the barrier layer of any electrochemicalpower sources can be modified. The power sources can be non-hermeticallysealed. The power sources can include metal-air batteries (such aszinc-air batteries, aluminum-air batteries, and magnesium-airbatteries), air recovery batteries, and air depolarized cells. Examplesof these power sources are described in U.S. Patent Nos. 2,597,119;3,436,270; U.S. Ser. No. 09/494,586, filed Jan. 31, 2000, now U.S. Pat.No., 6,432,438; and U.S. Ser .No. 09/544,076, filed Apr. 6, 2000, nowU.S. Pat. No. 6,399,243, all hereby incorporated by reference. The powersources can be those that include non-circular air access openings, suchas slits, slots and louvers, as described in U.S. Pat. No. 6,232,007;and U.S. Ser .No. 09/773,962, filed Feb. 1, 2001, and entitled“Battery”, now abandoned, all hereby incorporated by reference. Thepower sources can be a fuel cell.

The modification of the barrier layer can be applied to electrochemicalpower sources of various configurations. For example, the power sourcescan be a cylindrical cell, button cell, a prismatic cell of any crosssection, or a racetrack-shaped cell.

For non-hermetically sealed electrochemical power sources, the surfacearea or volume of portion(s) of the barrier layer that are modified canvary. The modified portions can be adjacent to air access openings onthe power sources, e.g., aligned with openings in a cell can. The areaof a modified portion can be equal to the area of an air access opening,or the areas can be different. For example, relative to the area of anair access opening, the area of a modified portion of a barrier layercan be greater by two times, four times, six times, eight times, sixteentimes, thirty-two times, or sixty-four times. In other embodiments,relative to the area of a modified portion of the barrier layer, thearea of the air access opening can be greater by two times, four times,six times, eight times, sixteen times, thirty-two times, or sixty-fourtimes. The areas can be substantially aligned or misaligned.

The barrier layer can be modified as described herein, e.g., by usingmechanical and/or chemical methods, and/or by changing the thickness ofthe barrier layer. In some embodiments, the barrier layer can bemodified after the power source has been assembled. For example, thebarrier layer can be mechanically worked by thumping or depressing anexterior portion of a battery can, which can spring back to positionafter thumping.

In other embodiments, the degree that portions of a barrier layer aremodified can vary. Some modified portions may have different porosity,density, and/or mass transport resistance than other modified portions.Modification of a barrier layer described herein, including as methods,degrees and locations of modified areas, can also be applied to aseparator layer between a cathode and an anode, e.g., to affect waterand/or electrolyte diffusion, and/or to the absorbent layer with ahydrophobic vapor barrier described above. In other embodiments, insteadof or in addition, to modifying certain areas to increase their masstransport resistance, other areas can be modified to relatively decreasetheir mass transport resistance, e.g., by using a thinner or less porousor less dense portions of barrier layer or separator.

Air Access Opening Design

As discussed above, at the pack level, the mass transport resistance ofcartridge 100, e.g., to oxygen, is in part a function of the pattern ofthe air access holes 112, e.g., their placement and/or area. The sizeand/or the location of the holes are preferably designed to maximizeoxygen transport while minimizing water transport. Also, the oxygen fluxacross the cathode in the pack should be uniform to maximize theutilization of active materials. For example, if there are too manyholes or if the holes are placed in less preferred positions, theingress or egress of water from the pack can cause premature failure. Ifthere are not enough holes positioned at preferred locations to supplysufficient oxygen, the pack may not be able to provide the, requiredcurrent for a given application.

Using finite element modeling of gas transport, including variables suchas the number of holes and the locations of the holes, certain patternsor designs of air holes are calculated to result in a decrease in therate of the water ingress into or egress from the cell, while providingsufficient oxygen transport for a selected application. For a three-celltelecommunications pack, for example, a design in which the air holesare placed between the cells, e.g., in the gap between the cells,results in a reduction of water transport by a factor of 3, while at thesame time maintaining the requisite oxygen concentration, e.g., for theCDMA or the GSM rate.

Model simulations revealed that, in packs 100, most of the watertransport was from the cathode directly underneath the air access holes.As holes are added directly over the cells, water transport continues toincrease. It is believed that the mechanism for water vapor transport isdiffusion, so concentration gradient is the driving force. Here, themajor concentration gradient is at the top of the cells, so watertransport occurs across the top of the cells. In comparison, theconcentration gradients at the sides and bottom of the cells arerelatively small, so the driving force for water transport across thesides and bottom is relatively small. Thus, forming holes directly overcells 104 markedly increases the rate of water transport. In somecircumstances, removing the holes that are directly above the top of thecells and using only those that are above the cell gaps reduced the rateof water loss/gain from the cells by approximately 60%.

Furthermore, while the oxygen partial pressure was the highest directlybelow the holes, forming holes directly over cells 100 did not markedlyenhance oxygen access. Oxygen access to bottom of the cells was also notenhanced by placing holes directly over the cells. Indeed, in certaincircumstances, removing holes from directly the top of cells did notaffect oxygen transport by much, e.g., less than about 1-2%. Thus, it isbelieved that holes placed over cell gaps in pack 100 can alone provideuniform and requisite oxygen distribution, for example, at GSM and/orCDMA rates, while water flux is minimized.

The diameter of the air access holes can also be designed to affect theperformance of pack 100 or an electrochemical cell, e.g., a metal-aircell (Example 3). In embodiments, as the hole diameter increases, thelimiting current increases and the shelf-life decreases. The limitingcurrent can increase relatively more than the decrease in shelf life.Increasing the hole diameter can also decrease the ratio of maximumcurrent density to minimum current density (Max/Min ratio), and thestandard deviation of the reaction current density. It is believed thatthe Max/Min ratio is an indicator of uniformity of flooding or wetting,and the standard deviation is an indicator of anode utilization. Thatis, decreasing the hole size deteriorates the standard deviation and theMax/Min ratio, and adversely affects utilization and wetting. Thus,decreasing the hole size appears to have the same effect as modifyingthe barrier layer as described above, in terms of decreasing limitingcurrent and increasing shelf-life. Accordingly, in some embodiments,design of the air access holes can be used in combination with themodified barrier layer described above. For example, thumped Teflon canbe used for the top sides of the cells to further reduce water flux inaddition to designing the air access holes only in the cell gaps. Byusing thumped Teflon directly underneath the air access holes,improvements can be achieved in reducing the non-uniformity in currentdensity and improving the shelf life of the packs.

The examples given below provide some methods for designing the airaccess openings. In some embodiments, the openings are preferably placedover the gap between adjacent cells. Rows of openings can be positionedsuch that they are centered over a sidewall of a cell. The openings canbe off-centered from the sidewall of a cell. For example, only aportion, e.g.; 10%, 20%, 30%, 40%, 60%,.70%, 80%, or 90%, of eachopening is over a sidewall of a cell. In some embodiments, no air accessopenings are directly and/or completely over the cells. The row(s) canbe completely and/or directly over the gap between adjacent cells. Thenumber of adjacent rows of openings near the gap can vary, e.g., 1, 2,3, 4, 5 or greater. Adjacent rows are preferably evenly spaced, but theycan be unevenly spaced. The spacing between adjacent openings can beeven or uneven. Openings can be non-circular, e.g., oval, elongated,and/or slits.

Metal-air Cell

Referring to FIGS. 6-11, a metal-air cell 104 includes a cathodeassembly 22, a bottom end member 24 connected to an end of assembly 22,and a top end member 26 connected to another end of assembly 22. Cathodeassembly 22 includes a cathode 28 formed on a current collector 30, aseparator 32 glued to an interior side of cathode 28, and a barrierlayer 34 wrapped around, e.g., laminated to, an exterior side of cathode28. Cathode assembly 22 is, formed to define a cavity 36. Cell 104further includes an anode 38 and an anode current collector 40 disposedin cavity 36. Anode current collector 40 extends from bottom end member24, through cavity 36, and through top end member 26. A negative tab 42is connected to anode current collector 40, and a positive tab 44 isconnected to cathode current collector 30. When cell 104 is fullyassembled (FIG. 1), barrier layer 34 defines an exterior surface of thebattery.

Cathode assembly 22 includes cathode 28 formed on current collector 30,separator 32, and barrier layer 34.

Cathode 28 includes an active cathode mixture having a catalyst forreducing peroxide, such as a manganese compound, carbon particles, and abinder. Useful catalysts include manganese oxides, such as Mn₂O₃, Mn₃O₄,and MnO₂, that can be prepared, for example, by heating manganesenitrate or by reducing potassium permanganate. Cathode 28 includesbetween about 1% and about 10%, preferably between about 3% and about 5%of catalyst by weight.

The carbon particles are not limited to any particular type of carbon.Examples of carbon include Black Pearls 2000, Vulcan XC-72 (Cabot Corp.,Billerica, Mass.), Shawinigan Black (Chevron, San Francisco, Calif.),Printex, Ketjen Black (Akzo Nobel, Chicago, Ill.), and Calgon PWA(Calgon Carbon, Pittsburgh, Pa.). Generally, the cathode mixtureincludes between about 30% and about 70%, preferably between about 50%and about 60%, of total carbon by weight.

Examples of binders include polyethylene powders, polyacrylamides,Portland cement and fluorocarbon resins, such as polyvinylidene fluorideand polytetrafluoroethylene. An example of a polyethylene binder is soldunder the tradename Coathylene HA-1681 (Hoechst). A preferred binderincludes polytetrafluoroethylene (PTFE) particles. Generally, thecathode mixture includes between about 10% and 40%, preferably betweenabout 30% and about 40%, of binder by weight.

The cathode mixture is formed by blending the catalyst, carbon particlesand binder, and is then coated on cathode current collector 30, such asa metal mesh screen, to form cathode 28. After the cathode mixture hashardened, cathode 28 is heated to remove any residual volatiles.

On the interior side of cathode assembly 22, separator 32 is glued tocathode 28. Separator 32 can be a porous, electrically insulatingpolymer, such as polypropylene, that allows electrolyte (describedbelow) to contact cathode 28.

On the exterior side of cathode assembly 22, barrier layer 34 islaminated to cathode 28 to complete cathode assembly 22. Barrier layer34 is air-permeable and liquid-impermeable. Layer 34, e.g., a PTFEmembrane, helps maintain a consistent humidity level in cell 104. Layer34 also helps to prevent the electrolyte from leaking out of the batteryand CO₂ from leaking into the cell.

Anode gel 38 contains a mixture of zinc and electrolyte. The mixture ofzinc and electrolyte can include a gelling agent that can help preventleakage of the electrolyte from the cell and helps suspend the particlesof zinc within the anode.

The zinc material can be a zinc powder that is alloyed with lead,indium, aluminum, or bismuth. For example, the zinc can be alloyed withbetween about 400 and 600 ppm (e.g., 500 ppm) of lead, between 400 and600 ppm (e.g., 500 ppm) of indium, or between about 50 and 90 ppm (e.g.,70 ppm) aluminum. Preferably, the zinc material can include lead, indiumand aluminum, lead and indium, or lead and bismuth. Alternatively, thezinc can include lead without another metal additive. The zinc materialcan be air blown or spun zinc. Suitable zinc particles are described,for example, in U.S. Ser. No. 09/156,915, filed Sep. 18, 1998, now U.S.Pat. No. 6,521,378, U.S. Ser. No. 08/905,254, filed Aug. 1, 1997, nowU.S. Pat. No. 6,284,410, and U.S. Ser. No. 09/115,867, filed Jul. 15,1998, now abandoned, each of which is incorporated by reference in itsentirety.

The, particles of the zinc can be, spherical or nonspherical. Forexample, the zinc particles can be acicular in shape (having an aspectratio of at least two). The zinc material includes a majority ofparticles having sizes between 60 mesh and 325 mesh. For example, thezinc material can have the following particle size distribution:

-   -   0-3 wt % on 60 mesh screen;    -   40-60 on 100 mesh screen;    -   30-50 wt % on 20.0 mesh screen;    -   0-3 wt % on 325 mesh screen; and    -   0-0.5 wt % on pan.

Suitable zinc materials include zinc available from Union Miniere(Overpelt, Belgium), Duracell (USA), Noranda (USA), Grillo (Germany), orToho Zinc (Japan).

The gelling agent is an absorbent polyacrylate. The-absorbentpolyacrylate has an absorbency envelope, of less than about 30 grams ofsaline per gram of gelling agent, measured as described in U.S. Pat. No.4,541,871, incorporated herein by reference. The anode gel includes lessthan 1 percent of the gelling agent by dry weight of zinc in the anodemixture. Preferably the gelling agent content is between about 0.2 and0.8 percent by weight, more preferably between about 0.3 and 0.6 percentby weight, and most preferably about 0.33 percent by weight. Theabsorbent polyacrylate can be a sodium polyacrylate made by suspensionpolymerization. Suitable sodium polyacrylates have an average particlesize between about 105 and 180 microns and a pH of about 7.5. Suitablegelling agents are described, for example, in U.S. Pat. NOS. 4,541,871,4,590,227, or 4,507,438.

In certain embodiments, the anode gel can include a non-ionicsurfactant. The surfactant can be a non-ionic phosphate surfactant, suchas a non-ionic alkyl phosphate or a non-ionic aryl phosphate (e.g.,RA600 or RM510, available from Rohm & Haas) coated on a zinc surface.The anode gel can include between about 20 and 100 ppm of the surfactantcoated onto the surface of the zinc material. The surfactant can serveas a gassing inhibitor.

The electrolyte can be an aqueous solution of potassium hydroxide. Theelectrolyte can include between about 30 and 40 percent, preferablybetween 35 and 40 of potassium hydroxide. The electrolyte can alsoinclude between about 1 and 2 percent of zinc oxide.

End members 24 and 26 are made of electrically-insulating materials,such as acrylonitrile-butadiene-styrene (ABS). Each member 24 and 26includes an opening sized to receive anode current collector 40. Bothend members 24 and 26 also include steps 45 and 49, respectively, tohelp align cathode assembly 22 with the end members, as described below.Furthermore, top end member 26 also defines a notch 47 through whichpositive tab 44 extends (FIG. 6).

Anode current collector 40, positive tab 44, and negative tab 42 aremade of materials that are stable to chemicals and electrical potentialspresent in cell 104. For example, anode current collector is made ofbrass; positive tab 44 is made of nickel metal; and negative tab 42 ismade of tin-plated brass.

To assemble cell 104, a blank, such as a rectangular sheet, of desireddimensions is punched or cut from a larger sheet of cathode assembly 22(made of cathode 28, current collector 30, separator 32, and barrierlayer 34). With portions of barrier layer 34 and separator 32temporality peeled back sufficiently, cathode material 28 is removed(e.g., by scraping) from both sides of opposing edges of cathodeassembly 22 to expose two areas of current collector 30. These exposedareas will be welded together after cathode assembly 22 is shaped.Cathode assembly 22 is then shaped to define cavity 36 and to overlapthe exposed areas of current collector 30 together. For example, thecathode assembly can be folded to form a cathode tube with a rectangularor square cross section, or can be bent or curled around a mandrel toform a cathode tube with a circular cross section. The exposed areas ofcurrent collector 30 are overlapped and welded together to provide arelatively rigid tube of cathode assembly 22. Parts of separator 32 thatwere temporarily peeled back are repositioned over a seam produced bywelding, overlapped and secured with a few drops of epoxy to separatecathode 28 and anode 38.

Positive tab 44 is then welded to current collector 30 on the exteriorside of cathode assembly 22 along the seam. A strip of sealant 46, suchas an epoxy (available from 3M, St. Paul, Minn.), is then applied overpositive tab 44 and the seam, along the length of cathode assembly 22 tominimize electrolyte and anode 38 from leaking from cell 104.

Anode current collector 40 is inserted through the opening in bottom endmember 24. Epoxy 48 is applied to on the exterior side of the opening ofbottom end member 24 to secure anode current collector 40 to the bottomend member and to minimize electrolyte and anode 38 from leaking throughthe opening.

Bottom end member 24 is then connected to cathode assembly 22 byinserting anode current collector 40 into cavity 36, and mating thecathode assembly with the bottom end member. Step 45 helps to centercathode assembly 22 on bottom end member 24. A bead of epoxy 50extending around cathode assembly 22 secures the cathode assembly tobottom end member 24 and minimizes leaks from cell 104. In someembodiments, bottom end member 24 can include a groove in which cathodeassembly 22 can sit, and bead of epoxy 50 can be deposited in thegroove.

Cavity 36 is then filled with anode 38.

A layer of epoxy 56 is applied to the interior surface of top end member26 to minimize leaks from cell 104. Top end member 26 is then placed oncathode assembly 22 such that notch 47 receives positive tab 44 andanode current collector 40 extends through the opening in the top endmember. Step 49 helps to center cathode assembly 22 on top end member26. By extending anode current collector 40 through both end members 24and 26, the anode current collector acts as a rigid support thatenhances the mechanical integrity of cell 104. Also, an anode currentcollector that extends along the length of cell 104 can also optimizethe efficiency of discharge by providing a relatively small voltagedrop. A bead of epoxy 52 is applied to top end member 26, around cathodeassembly 22, similar to epoxy 48. Epoxy 54 is also applied to theexterior side of the opening of top end member 26 to secure anodecurrent collector 40 to the top end member and to minimize electrolyteand anode 38 from leaking through the opening.

Negative tab 42 is then welded to anode current collector 40 to completeassembly of cell 104.

Cells 104 are placed in pack 100 with their negative and positive tabsappropriately connected, e.g., in series (FIGS. 10 and 11).

Numerous other combinations of steps can be used to assemble cell 104.For example, anode current collector 40 can be glued to top end member26 and welded to negative tab 42, and this assembly can be connected asone unit to cathode assembly 22. Bottom end member 24 may not include anopening. Anode current collector 40 may extend only a portion of thelength of battery 20. Tabs 42 and 44 may extend from cell 104 toopposite ends of the battery. Cell 104 may not include a negative tab.End members 24 and 26 and/or cathode assembly 22 l may have differentcross section shapes, such as triangular, circular, square, andrectangular. Cell 104 can be formed in numerous sizes, such as, forexample, to fit inside and to be used with a metal-air cartridge asdescribed in commonly-assigned U.S. Ser .No. 09/693,010, now U.S. Pat.No. 6,384,574.

In other embodiments, separator 32 and barrier layer 34 can be disposedadjacent to cathode 28 by other means. For example, separator 32 can becast from solution and be formed on cathode 28 when the solution dries.

The following examples are illustrative and not intended to be limiting.

EXAMPLE 1

Applying finite element modeling generally includes solving equations(given below) for a specified geometry of a system. The solutionprocedure generally includes entering the geometry into FEMLAB/MATLABsoftware, entering the equations at each boundary in the geometry, andsolving the equations using the built-in algorithm for Finite Elementsolver in the software. The plenum is divided into a mesh of between3,000 and 19,000 elements.

The governing equations include:Mass conservation (bulk): ∇·ν=0Mass conservation (species): ρν·∇X _(m)+∇·(−Dρ∇X _(m))=0Momentum conservation: ρν·∇ν_(x)+∇·(−μ∇ν_(x))+δp/δx=0where ν=velocity; ρ=density; and X=mass fraction. The boundaryconditions cover the air access holes, the walls, and the cathode. Forthe air access holes, ρ, X_(m), and p are all ambient. Velocity (ν) isproportional to applied current. For the walls, normal fluxes are zero(bulk mass, species, momentum). For the cathode, n·(−Dρ∇X_(m)) isproportional to j, the current density. Current density (j) isproportional to ρX_(m).

The oxygen concentration, [O₂], is solved for [O₂]=ρX_(m), normalized toambient.

EXAMPLE 2

This example condenses 3-D finite element modeling results to anempirical algorithm that contains a set of empirical equations andconditions. The algorithm can provide a general framework that aids inthe finding of a preferred air hole access design, such as to obtain apreferred number of holes and their placement during design of a batterycartridge or pack. The algorithm is valid for any arbitrary prismaticpack design. The algorithm also presents a method to account formechanical constraints that may arise during the design of the packs.This approach can result in time savings and independence from runningfinite element simulations every time a change is made in the design.FIG. 15 shows a flowchart summarizing the algorithm for design and theconditions under which it is valid.

The algorithm is general and can be used for any cell-pack with nprismatic canless cells. Referring to, FIGS. 12 and 13, the cellparameters are: the cell width (x); the cell, height (y); the tab width(z); and the active cell length (l). The plenum parameters are: theinter-cell plenum width (w); the plenum depth on the top (p); and theplenum depth on the bottom (b). The hole pattern parameters are: thehole diameter (d); the number of columns over each inter-ell plenum (c);and the number of rows of holes (r). The algorithm can be used todetermine d, c and r using input parameters x, y, z, l, w, p and b, forexample, required to sustain a CDMA or a GSM rate in metal-air packs.The total number of holes is rc(n+1).

Generally, the lowest oxygen concentrations are at the bottom of thecell since this is the longest diffusion path length (from the airaccess holes to the bottom of the cells). Therefore, the tabbed side ofeach cell, which is relatively electrochemically inactive, is preferablyplaced at the bottom of the cartridge to provide an increase in theaverage oxygen concentration at the other cathode surfaces.

Finite element analysis indicates that the openings are preferablyplaced over the plenum between the cells, e.g., not directly over thecells. Such placement minimizes the difference between the resistancesto diffusion from the openings to the top and to the sides and/or bottomof the cells, so long as the resistance is greater for diffusion to thebottom than to the top. This condition can be formulized as follows:

$\begin{matrix}{\frac{x}{2p} < {\frac{2y}{w} + \frac{x - z}{2b}}} & (1)\end{matrix}$Thus, in a pack design for which inequality (1) holds, the openings arepreferably placed over the inter-cell plenum. If mechanicalconsiderations preclude the placement of openings over the plenumbetween the end cell and the pack wall, then openings may be placed overthe end cell, as close as possible to the plenum. Except for the endcells, placement of openings directly over cells can exacerbate waterloss without significantly improving air access.

In some embodiments, a mechanical constraint is set such that when thepack is closed, the center-to-center distance between correspondingcircular holes in each sheet is approximately twice the hole diameter.Since the distance from hole to hole between sheets is 2d, the distancefrom hole to hole within a sheet should be 4d in the: lengthwisedirection. Also, if multiple columns of holes are used over eachinter-cell plenum (i.e., c>1), the center-to-center distance betweencolumns should be 2d.

A hole pattern based solely on maximizing the inter-cell open area wouldhave a single column, of large holes (i.e., c=1) over each plenum, so asto make more efficient the space between columns. In such a design, dwould be equal to the inter-cell width, w. Therefore, the lengthwisecenter-to-center distance would be at least 4w. However, simulationshave shown that the farther apart the openings are in the lengthwisedirection, the worse the oxygen access is to points, farthest from theopenings. In particular, 4w can be too far, so a compromise is preferredbetween the amount of open area and its distribution in the lengthwisedirection.

Referring to FIG. 14, the minimum normalized oxygen concentrationgenerally increases with an increase in c. The increase in theconcentration with an increase in c from 1 to 2 is approximately 15% ofthe c=1 value. With an increase in c from 2 to 3 and 4 the increase inthe minimum concentration is approximately 4% of the c=2 value.Therefore, the benefit from increasing c from 1 to 2 is significant andthe benefit decreases when c is increased beyond 2. From both amanufacturing standpoint and a seal leakage standpoint, there can behigher cost and higher chance of failure as more holes are formed. Thus,c=2 represents a preferred number of columns over each inter-cellplenum. Simulations have shown that the c=2 preference is independent ofthe plenum width, w.

Finite element analysis has also demonstrated that the two columnsshould be centered over the cell/plenum interfaces. In this case, withthe constraint that the center-to-center distance between columns is atleast 2d, the hole diameter preferably obeys the following inequality:

$\begin{matrix}{d \leq \frac{w}{2}} & (2)\end{matrix}$

It is not evident a priori whether d should in fact be equal to or lessw/2. If d=w/2, then the center-to-center distance between columns isequal to 2d and the open area is maximized. However, the lengthwisedistance between holes, 4d, is also maximized. Simulations have shownthat for a given c, the benefits gained by maximizing the open areaslightly outweigh detriments from unpreferred lengthwise distribution.Thus, the preferred diameter is given as:

$\begin{matrix}{d = \frac{w}{2}} & (3)\end{matrix}$

If the center-to-center distance between holes in a column is ad, wherea is approximately four, then the number of rows of holes is given by:

$\begin{matrix}{r = \frac{l}{ad}} & (4)\end{matrix}$where l is the active cell length. Equations 3 and 4 along with c=2, canbe used to determine a preferred hole pattern for a given w.

If in designing a hole pattern for a given pack design, the pattern ofholes given by these formulas is not mechanically feasible, then somemodifications can be made to the pattern. Simulations have shown thatthe positions of the columns in the widthwise direction, the distancebetween them, the hole diameter (d), and/or the parameter (a) may beadjusted. In adjusting d and a, it is preferable to satisfy Equation 4(that is, r is preferably changed accordingly), but Inequality 2 may besubstituted for Equation 3. If a feasible hole pattern cannot beobtained by making small changes in these four parameters, then choosinga different value of c may help.

When new packs are designed, the above process can be used to predict apreferred hole pattern rather than iteratively simulating changes to oldhole patterns. The process is shown in FIG. 15. The preferred number ofcolumns (c) over each inter-cell plenum is set at two, and Equations 3and 4 are used to determine the preferred values of the hole diameter(d) and the number of rows (r). If the hole pattern given by d, c, and tis feasible, then it can be used. If not, adjustment may be used asdescribed above to design a feasible hole pattern. If possible, the samesize, number, and pattern of holes is preferably placed over each endcell as over each inter-cell plenum, and the holes over each end-cell ispreferably as close as possible to the pack wall. Hole patterns designedaccording to this process can maximize air access and minimize waterloss while substantially maintaining the mechanical constraints placedon the cover design.

EXAMPLE 3

This example investigates the use of thumped Teflon in button cells torender the current density in the cathode more uniform and to improvethe activated shelf life. Using the finite element gas transport modeldescribed above, the effect of thumping an unlaminated Teflon layer in a9-hole 675 Duracell zinc-air button cell was investigated.

Selective thumping of the unlaminated Teflon disc in button cells can beused to significantly extend the activated shelf life and also improvethe uniformity in the current distribution in the cathode. For example,thumping of unlaminated Teflon in circular segments with 4× the diameterof the air access holes directly beneath the holes results in theextension of the shelf life approximately by a factor of 2. Also, thecurrent density is 5-10 percent more uniform. A more uniform currentdensity can result in more uniform flooding and improved anodeutilization, both highly desirable for a button cell performance.However, these advantages can be at the cost of reduced limiting currentof the cell.

The MEV675 button cell that was evaluated has nine air access holes inthe can: eight holes evenly spaced around a circle, and one hole in thecenter of the circle. Diffusion in a 45° pie slice, containing one ofthe peripheral holes and one eighth of the center hole, was considered.Diffusion was modeled through a Wattman paper air dispersion disc ofporosity 0.5, a Teflon disc of porosity 0.2, and a laminated Teflonlayer (on the cathode) also of porosity 0.2. The effect of thumping onthe current distribution and the shelf-life were simulated and comparedto an unthumped cell.

Four different thumping patterns were simulated to get a general senseof how thumping affects oxygen diffusion (FIGS. 16A-16D). Referring toFIG. 16A, in pattern 16A only the area directly under each hole wasthumped. Thus, pattern 16A includes nine circular regions, each with adiameter equal to the diameter of the air access holes. Similarly,pattern 16B (FIG. 16B) includes nine circular regions centered under theair access holes, but each thumped region has a diameter four timesgreater than the diameter of the air access holes. The thumped area inpattern 16B is sixteen times greater than the thumped area in pattern16A. Patterns 16C and 16D each includes one circular thumped regionunder the center hole and one ring-shaped, thumped band under the eightperipheral holes (FIGS. 16C and 16D). In pattern 16C, both the diameterof the circular thumped region and the width of the thumped band areequal to the air-access hole diameter. In pattern 16D, both the diameterof the circular thumped region and the width of the thumped band arefour times greater than the air-access hole diameter. In some cases,patterns 16A and 16B can be relatively more rational from a currentdistribution standpoint, whereas patterns 16C and 16D may be moreconvenient from a manufacturing standpoint. Various final porositiesbetween 0 and 0.2 were simulated for each thumping pattern.

A 3-D finite element mesh was generated on the three layers using FEMLAB(FIG. 17). The, 3-D steady-state diffusion equation was numericallysolved on this mesh for the various thumping patterns, final porosities,and boundary conditions. The effective diffusion coefficient in eachsubdomain (Wattman Paper, unthumped Teflon, and thumped Teflon) wasrelated to the layer porosity, ε, according to the equationD_(eff)=Dε^(1.5). The oxygen concentration at the air-access holes wasassumed to be ambient. In simulations used to determine the limitingcurrent, the oxygen concentration at the cathode/laminated Tefloninterface was set to zero. The flux was then integrated to determine thelimiting current. The limiting current in the unthumped case was alsoapproximated experimentally via a series of galvanostatic discharges.For the unthumped case, the measured value of 30 mA was half thepredicted value of 60 mA. It is believed that this discrepancy is due toinaccuracy in the value of the diffusion coefficient, D, and/or themoles of electrons per mole of oxygen, n. Halving either D or n gives anaccurate limiting current. Since D and n have equivalent effects for allsimulations, either parameter can be halved.

Normalized oxygen partial pressure maps were solved at an appliedcurrent of 5 mA (using the experimentally corrected D or n) for severalrepresentative thumping patterns. FIGS. 18A and 18B show sample oxygendistributions in the unthumped and thumped cases, respectively. As seenfrom FIGS. 18A and 18B, the oxygen partial pressure directly below theair access hole decreases with thumping. This results in a decrease inthe non-uniformity in the current distribution.

FIG. 19 shows the effects of thumping on the limiting current. Thelimiting current, normalized to the unthumped case, is plotted versusfinal porosity for all four thumping patterns. In each case, thelimiting, current decreases almost linearly as porosity is decreased.The patterns with more area thumped show a greater decrease in limitingcurrent. The greatest decrease shown is for pattern 16D, thumped to0.01% porosity. In this case, the limiting current is half that of theunthumped case.

Activated shelf life is inversely proportional to limiting current.Thus, limiting current can be used to predict the effect of thumping onactivated shelf life. FIG. 20 shows the ratio of thumped shelf life tounthumped shelf life plotted versus final porosity. The shelf lifeincreases as porosity is decreased. The patterns with more area thumpedshow a greater increase in activated shelf life. In pattern 16D, thumpedto 0.01% porosity, the activated shelf life is twice that of theunthumped case.

Both the standard deviation and the range (minimum and maximum values)of local current density are considered indicators of currentdistribution uniformity. In a more uniform current distribution, thestandard deviation is relatively low and the range is relatively narrow.The standard deviation of current density was roughly the same for allcases considered, with no significant trends observed. The minimum localcurrent density at an applied current of 5 mA was also the same for allcases considered. However, significant variation was observed in themaximum local current density.

Referring to FIG. 21, the ratio of maximum local current density to meanlocal current density is plotted for several cases. In the unthumpedcase, the maximum is 1.15 times the mean. Thumping pattern 16A showed noeffect on this maximum. In thumping pattern 16B, however, where 16 timesthe area, under each hole was thumped, a significant decrease in themaximum local current density at 5 mA is observed. This narrowing of therange becomes more pronounced as the final porosity is decreased. Theratio of maximum to mean goes to 1.08 as the final porosity approaches0. Although pattern 16D also shows a narrowing of the range, its effectis not as significant as that of pattern 16B. In pattern 16D, thethumping is not symmetric with respect to the holes. This asymmetrycontributes to current non-uniformity.

Selective thumping of the Teflon disc can be used to significantlyextend the activated shelf life. In these illustrative simulations,thumping can extend the shelf life to twice that of the unthumped case.The smaller the final porosity and the greater the thumped area, thegreater the increase in shelf life. This improvement in shelf life comesat the cost of decreased limiting current, because shelf life isinversely proportional to limiting current. Thus, in choosing a thumpingpattern, the maximum current to be sustained by the cell should bedetermined. For maximum activated shelf life, a thumping pattern andporosity can then be chosen to give a limiting current somewhat higherthan this maximum current.

Furthermore, as seen from FIG. 19, there are multiple thumping patternsand porosities that will provide the same limiting current. For example,if a limiting current equal to 80% that of the unthumped case isdesired, pattern 16C thumped to 2% porosity and patterns 16B and 16Dthumped to 11% porosity can all be effective. In fact, although, onlyseveral patterns were simulated here, there are an infinite number ofconceivable patterns and porosities that would provide the desiredlimiting current. To choose which of these is preferable, bothmanufacturing and current distribution should be considered. Withrespect to current distribution, in some circumstances, patterns inwhich areas of high local current density (in the unthumped case) arethumped will be optimal. Such patterns can shift some of the load fromthese regions to other regions, thus smoothing out the currentdistribution. The areas of highest current density (in the unthumpedcase) are right under the holes, with the current density decreasinggradually as distance from the holes is increased. To a firstapproximation, the optimal thumping pattern should therefore includecircular regions (of size and porosity determined by the desiredlimiting current) centered on the holes, as in patterns 16A and 16B. Ifthis is not feasible from a manufacturing standpoint, other patterns,such as 16C and 16D, may be utilized. In some cases, the first prioritymay be to choose the appropriate limiting current, thereby optimizingthe shelf life, and the second priority may be to choose a thumpingpattern that will provide such a limiting current and also smooth outthe current distribution within manufacturing constraints.

As discussed above, the dimensions of the air access openings can bedesigned, e.g., increased or decreased, to affect a cell's performance.FIG. 22 shows plots of standard deviation and Max/Min as a function ofincreases in air access opening diameter. FIG. 23 shows plots of changesin limiting current and shelf life, and changes, in current densitystandard deviation as a function of changes in air access openingdiameter. FIG. 24 shows plots of changes in activated life and changesin limiting current as a function of changes in air access openingdiameter. The barrier layer was modified to 5% final porosity. Themodified pattern was similar to pattern 16A in which only the areadirectly under each hole was modified, e.g., thumped.

All publications and patents mentioned in this application are hereinincorporated by reference to the same extent as if each individualpublication or patent was specifically and individually indicated to beincorporated by reference.

Other embodiments are within the claims.

1. A battery, comprising: a cathode comprising an active cathode mixturecoated on a cathode current collector arranged as a tube having twolongitudinally-extending ends that have been connected together to forma seam, the cathode having an interior surface and an exterior surface,the cathode defining a cavity and two open ends; alongitudinally-extending positive tab connected to the cathode currentcollector; a separator disposed adjacent to the interior surface of thecathode; an anode disposed adjacent to the separator and inside thecavity; an air-permeable, liquid-impermeable barrier layer disposedadjacent to the exterior surface of the cathode, the barrier layerdefining an exterior surface of the battery; two end members connectedto the open ends of the cathode; and an anode current collectorextending through the two end members wherein the positive tab isconnected to the current collector along the seam; and furthercomprising a longitudinally-extending strip of sealant disposed over thepositive tab.
 2. The battery of claim 1, wherein the barrier layercomprises polytetrafluoroethylene.
 3. The battery of claim 1, whereinthe end members are configured to mate with the cathode and align withthe cathode along a longitudinal axis of the battery.
 4. The battery ofclaim 1, further comprising a conductive tab connected to the anodecurrent collector.
 5. The battery of claim 1, wherein the end membersand the cathode are connected by a sealant.
 6. The battery of claim 1,wherein the end members comprise an electrically-insulating material. 7.The battery of claim 1, wherein the cathode has a substantiallyrectangular cross section.
 8. The battery of claim 1, wherein thecathode has a substantially square cross section.
 9. The battery ofclaim 1, wherein the cathode has a substantially triangular crosssection.
 10. The battery of claim 1, wherein the cathode has asubstantially circular cross section.
 11. The battery of claim 1,wherein the battery is a metal-air battery.
 12. An article, comprising:a housing; and the battery of claim 1 inside the housing; wherein thehousing is adapted to manage air flow into and out of the housing.